1. Field of the Invention
The present invention relates generally to CDMA mobile communication systems, and more specifically to a diversity branch selecting method and system for a base station using a list of field intensity levels detected and sent from a mobile station. The present invention is particularly useful for diversity handover known as micro-diversity.
2. Description of the Related Art
In a CDMA mobile communication network, when a mobile station encounters a low signal quality on the current communication path (known as a primary diversity branch), it begins a search through a number of diversity branches to surrounding cell-sites or a number of sector-antenna elements for pilot signals and sends a report to the current cell-site on their power levels to request it to determine a second diversity branch in preparation for a possible inter-cell diversity handover or a possible intra-cell diversity handover.
According to the prior art, diversity branch signals are maximal-ratio combined in the case of an intra-cell diversity handover, while they are selectively combined on a frame-by-frame basis in the case of an inter-cell diversity handover. As a result of the different combining schemes, the diversity branch selected by the prior art is found to be not necessarily the best at all times. If the power levels of two pilot signals on uplink diversity branches are equal to each other, the maximal ratio combining of these signals produces an improvement of about 3 dB, while the improvement obtained by the frame-by-frame selective combining is as low as 1 dB. In so far as the quality of uplink signals is concerned, the difference of 2 dB is of a significant value. Therefore, if there is a difference of 1 dB between two pilot signals and one of these having lower quality is arriving from an intra-cell diversity branch (i.e., another sector of the same cell-site), the maximal-ratio combining of this signal with the current signal would produce an improvement greater than would be obtained by the frame-by-frame selective combining of the higher quality signal.
It is therefore an object of the present invention to provide a method and system for selecting a diversity branch that is optimum for both intra-cell maximal ratio diversity combining and inter-cell selective diversity combining.
According to a first aspect of the present invention, there is provided a method of selecting a diversity branch from a list of pilot signals detected by and sent from a mobile station and informing the mobile station of identity of the selected diversity branch as an additional diversity branch, the pilot signals indicating their power levels and identities of diversity branches through which the pilot signals are respectively transmitted to the mobile station, the method comprising the steps of (a) storing diversity gain data in a first memory which indicates an amount to be gained when maximal ratio combining is performed on signals from intra-cell diversity branches and storing diversity gain data in a second memory which indicates an amount to be gained when selective combining is performed on signals from inter-cell diversity branches, b) determining whether the diversity branch of each pilot signal of the list is an intra-cell diversity branch or an inter-cell diversity branch, (c) if the diversity branch of each pilot signal is the intra-cell diversity branch, reading a diversity gain from the first memory and summing the read gain with the power level of each pilot signal, (d) if the diversity branch of each pilot signal is the inter-cell diversity branch, reading a diversity gain from the second memory and summing the read gain with the power level of each pilot signal, and (e) selecting one of the diversity branches of the list corresponding to the highest of sums obtained by steps (c) and (d).
According to a second aspect, the present invention provides a method of selecting a diversity branch from a list of pilot signals detected by and sent from a mobile station and informing the mobile station of identity of the selected diversity branch as an additional diversity branch, the pilot signals indicating their power levels and identities of diversity branches through which the pilot signals are transmitted respectively to the mobile station, the method comprising the steps of (a) storing diversity gain data in a memory which indicates a difference between an amount to be gained when maximal ratio combing is performed on signals from intra-cell diversity branches and an amount to be gained when selective combining is performed on signals from inter-cell diversity branches, (b) determining whether the diversity branch of each pilot signal of the list is a predetermined one of an intra-cell diversity branch and an inter-cell diversity branch, (c) if the diversity branch of each pilot signal is the predetermined one, reading a diversity gain from the first memory and combining the read gain with the power level of each pilot signal to produce a sum, (d) if the diversity branch of each pilot signal is other than the predetermined one, identifying the power level of each pilot signal as a non-sum, and (e) selecting one of the diversity branches of the list corresponding to the highest of at least one sum produced by step (c) and at least one non-sum identified by step (d).
According to a third aspect, the present invention provides a system for selecting a diversity branch from a list of pilot signals detected by and sent from a mobile station and informing the mobile station of identity of the selected diversity branch as an additional diversity branch, the detected pilot signals indicating their power levels and identities of diversity branches through which the pilot signals are respectively transmitted to the mobile station. The system comprises a first memory for storing diversity gain data which indicates an amount to be gained when maximal ratio combining is performed on signals from intra-cell diversity branches, a second memory for storing diversity gain data which indicates an amount to be gained when selective combining is performed on signals from inter-cell diversity branches, and control means. The control means determines whether the diversity branch of each of the pilot signals is an intra-cell diversity branch or an inter-cell diversity branch, and reads a diversity gain from the first memory and summing the read gain with the power level of each pilot signal if the diversity branch is determined to be the intra-cell diversity branch, or reads a diversity gain from the second memory and summing the read gain with the power level of each pilot signal if the diversity branch is determined to be the inter-cell diversity branch, whereby a plurality of summed values are produced. The control means selects one of the diversity branches of the list corresponding to the highest of the summed values.
According to a fourth aspect, the present invention provides a system for selecting a diversity branch from a list of pilot signals detected by and sent from a mobile station and informing the mobile station of identity of the selected diversity branch as an additional diversity branch, the detected pilot signals indicating their power levels and identities of diversity branches through which the pilot signals are respectively transmitted to the mobile station. The system comprises a memory for storing diversity gain data which indicates a difference between an amount to be gained when maximal ratio combining is performed on signals from intra-cell diversity branches and an amount to be gained when selective combining is performed on signals from inter-cell diversity branches. Control means determines whether the diversity branch of each of the pilot signals is a predetermined one of an intra-cell diversity branch and an inter-cell diversity branch, reads a diversity gain from the memory, summing the read gain with the power level of each pilot signal if the diversity branch of the pilot signal is determined to be the predetermined one to produce a sum, identifies the power level of each pilot signal as a non-summed power level if the diversity branch of the pilot signal is determined to be other than the predetermined one, and selects one of the diversity branches of the list corresponding to the highest of a plurality of the summed power level and a plurality of the non-summed power level.
According to a fifth aspect of the present invention, there is provide a computer readable storage medium for a system including a first memory storing diversity gain data which indicates an amount to be gained when maximal ratio combining is performed on signals from intra-cell diversity branches and a second memory storing diversity gain data which indicates an amount to be gained when selective combining is performed on signals from intra-cell diversity branches, the storage medium storing instructions data which comprises the steps of (a) receiving a list of pilot signals detected by and sent from a mobile station, the pilot signals indicating their power levels and their diversity branches, (b) determining whether the diversity branch of each pilot signal of the list is an intra-cell diversity branch or an inter-cell diversity branch, (c) if the diversity branch of each pilot signal is the intra-cell diversity branch, reading a diversity gain from the first memory and summing the read gain with the power level of each pilot signal, (d) if the diversity branch of each pilot signal is the inter-cell diversity branch reading a diversity gain from the second memory and summing the read gain with the power level of each pilot signal, (e) repeating steps (b) to (d) until a plurality of summed values are produced from all pilot signals of the list, and (f) selecting one of the diversity branches indicated in the list having a highest value of the summed values.
According to a sixth aspect, the present invention provides a computer readable storage medium for a system including a memory storing diversity gain data which indicates a difference between an amount to be gained when maximal ratio combining is performed on signals from intra-cell diversity branches and an amount to be gained when selective combining is performed on signals from inter-cell diversity branches, the storage medium storing instructions data which comprises the steps of (a) receiving a list of pilot signals detected by and sent from a mobile station, the pilot signals indicating their power levels and their diversity branches through which the pilot signals are detected, (b) reading a diversity gain from the memory corresponding to a pilot signal of the list and summing the read gain with the power level of the pilot signal if the diversity branch of the pilot signal is a predetermined one of the intra-cell and inter-cell diversity branches and identifying the power level of the pilot signal as a non-sum if the diversity branch of the pilot signal is other than the predetermined one, (c) repeating step (b) on all pilot signals of the list to produce at least one sum and at least one non-sum, and (d) selecting one of the diversity branches of the list corresponding to the highest of sums and non-sums produced by step (c).